In recent years there has been an increased consciousness of energy use, the result of both higher energy costs and a desire for more sustainable systems. Lighting has long been a significant user of energy, and thus there has been an effort to develop more efficient lighting systems that produce more usable light for less energy.
The most desirable light is white light, since that is considered the brightest light having the most uses for home and commercial lighting as well as for military applications such as Navy installations, ships, and components.
One approach to achieving energy efficient white lighting has involved the use of light emitting diodes (LEDs). In some cases, white-light LEDs are formed by using three separate LED dies of different colors, which when mixed together create white light, for example, an InGaN blue LED, an InGaN greed LED, and an AlInGaP red LED. However green LEDs have a very low efficiency, which makes this an unsuitable solution from an energy-saving standpoint. In other cases, a blue or UV InGaN LED can be used, which is then down-converted to white light by use of an energy down-converting phosphor. See U.S. Pat. No. 6,600,175 to Baretz (white light emitter assembly consisting of a GaN light-emitting diode with a down-converting phosphor).
An important metric for a white light source is its luminous efficacy, which is measured in lumens per watt. Current production white LEDs have an efficacy of 60-70 lm W−1, which is greater than the high efficacy incandescent lamps (17 lm W−1) but only comparable to the efficacy of less-expensive compact fluorescent lamps (approximately 60 lm W−1) and linear fluorescent lamps (approximately 80 lm W−1).
Another approach to creating a white light source has been to substitute a GaN-based laser diode as the excitation source since once a drive current in a laser diode reaches a turn-on threshold, the light output increases linearly with drive current. See Xu et al., “Phosphor-conversion white light using InGaN ultraviolet laser diode,” Appl. Phys. Lett 92, 021129 (2008).
The efficiency and efficacy of traditional GaN-based LEDs and laser diodes are limited by the inherent physics of such devices. For example, like other lasers, laser diodes have a high current turn-on threshold to achieve the population inversion required for laser operation, and their high cost and low efficiency limits their desirability as a lighting solution. Although LEDs do not require the population inversion needed for laser diodes, the luminous efficacy of semiconductor LEDs is limited by the saturation of output power with increasing operating current from self-heating.
Thus, white-light LEDs and laser diodes cannot cost-effectively replace incandescent or fluorescent light sources until there is a significant decrease in their cost per lumen.
Polariton-based devices have been developed in recent years as another type of light-emitting device. See U.S. Pat. No. 5,877,509 to Pau et al. (describing a quantum well exciton-polariton light-emitting device).
A polariton is a hybrid quasi-particle consisting of both an exciton and a photon. The creation of a polariton requires strong coupling between a photon and exciton at the same energy. An exciton is a bound electron hole pair whose Coulombic interaction serves as the binding energy which is formed by optically and electrically pumping a semiconductor layer. In semiconductor optoelectronic devices, electrical bias applied to a pn junction to inject holes from the p-type layer and electrons from the n-type layer to the interface. Commonly, a quantum well or wells is designed at this interface to trap and thus increase the concentration of injection electrons, holes and excitons.
Gallium nitride (GaN) is often used as a material for such polariton devices because it has a large exciton binding energy (bulk=30 meV) that inhibits the dissociation of polaritons at room temperature and at high exciton densities. See N. Antoine-Vincent, “Observation of Rabi splitting in a bulk GaN microcavity grown on silicon,” Phys. Rev. B, 68, 153313 (2003). In general, a quantum well increases the intersection of the electron and hole wavefunctions, with the binding energy of the exciton, with energies of 50 meV reported for a GaN quantum well. See G. Weihs et al., “Exciton-polariton lasing in a microcavity,” Semicond. Sci. Tech. S386, 18 (2003); and P. A. Shields et al., “Magneto-photoluminescence of AlGaN/GaN quantum wells,” J. Cryst. Growth, 230, 487 (2001).
A resonant cavity photon mode is established by placing the quantum well at the anti-node of an optical cavity formed by a top and bottom mirror each typically composed of a semiconductor distributed Bragg refractor (DBR) or a dielectric DBR. A strong coupling condition is established when the optical resonance energy of the cavity is matched to the recombination energy of the exciton in the bound state. Under this condition, neither distinct photons nor excitons exist, rather an exciton-polariton hybrid particle with a lifetime less than the energy relaxation time for leakage of photons from the cavity. If the polariton has time to condense before it relaxes, then it is possible at low injection to emit a relatively intense coherent beam of light through the less reflective mirror, and this spontaneous emission of coherent light from exciton-polariton condensate creates a polariton laser. See S. Christopoulos et al., “Room-Temperature Polariton Lasing in Semiconductor Microcavities,” Phys. Rev. Lett. 98, 126405 (2005). The bound exciton associated with the polariton has a very short lifetime, and this short lifetime allows for very fast modulation of the light emission.